Vehicles such as aircraft have an electromechanical machine functioning as a generator to provide electrical power to equipment loads throughout the aircraft. The primary function of the generator is to generate electricity, but the electromechanical machine when coupled to an engine may also be used as a starting motor to start the engine. When operating as the generator, mechanical energy is converted into electrical energy using electromagnetic induction. Reverse conversion of electrical energy into mechanical energy to provide torque to start the engine is done by the electric starting motor. Therefore, one electromechanical machine can operate as both an electric starting motor to drive the engine up to the desired idle speed and as a generator to provide electrical power to the aircraft which helps to reduce the cost and weight of the aircraft.
Typically the parts of the electromechanical machine are expressed in either mechanical or electrical terms which may be used interchangeably or in combination. The two principal components are the rotor and stator where the rotor is the rotating part of the generator or the starting motor and the stator is the stationary part of the generator or starting motor. The rotor of the electromechanical machine rotates within a magnetic field produced by permanent magnets or by field coils where current must flow in the field coils to generate the magnetic field. The process of generating a magnetic field with an electric current is called excitation.
U.S. Pat. No. 4,743,777 to Shilling et al., which is hereby incorporated herein by reference in its entirety, teaches that the rotor of these starter generator systems carries a main field winding and a three phase exciter armature winding along with a rectifier for rectifying the output of the exciter armature winding to provide DC excitation for the rotating DC main field winding. The stator includes a main armature winding and an exciter section of two windings. The main armature winding is magnetically coupled to the main field winding on the rotor.
The exciter section of two windings on the stator includes a first exciter field winding and a second exciter field winding that are both mounted to be magnetically coupled to the exciter armature windings of the rotor. The first exciter field winding is a multiple phase AC exciter field winding and the second exciter field winding is a DC exciter field winding. During starter motor operation, the AC exciter field winding receives power from an external power source. During generator operation, DC power is supplied to the DC exciter field winding. However, synchronous brushless machines utilizing two exciter field windings have a high number of components and require complicated controls which hurt reliability. Use of two excited field winding also requires more space and weight which makes the manufacture and use of the aircraft less efficient. Also, the field voltage of the generator is either increasing or decreasing due to the effect of variation of back electro-motive-force (back EMF) resulted from changing speed of the rotor during the starter mode.
It is with respect to these and other considerations that the disclosure herein is presented.